1. Field of the Invention
The present invention relates to a roll bending device for forming plastic sheet or film (hereinafter called as "plastic sheet").
2. Description of the Related Art
FIGS. 2 to 4 explain a conventional roll bending device for forming plastic sheet.
FIGS. 2 and 3 is schematic views showing a deformation state of rolls in forming plastic sheet. As shown in FIG. 2, in a case where molten resin is guided to a pair of rolls to form plastic sheet, rolls 1 and 2 are subjected to counterforce based on flow stress, and curved. As a result, the formed plastic sheet has a substantially barrel-like cross section. Actually, in order to prevent the generation of such an improper cross section, one roll 2 is curved against the other roll 1 before the plastic sheet is formed. Thereby, the formed plastic sheet has a flat cross section.
FIG. 4 shows the structure of the conventional roll bending device. In this case, FIG. 4 illustrates only one side since this device is structured to be symmetrical in a width direction of the rolls.
A frame 6 for supporting the entire device including the rolls is provided at end portions of the rolls 1 and 2. A rotation shaft 25 is projected from the end portion of the upper side roll 1. The rotation shaft 25 is supported by a bearing 4. A housing 5 containing the bearing 4 is fixed to the frame 6. A rotation shaft 7 is projected from the end portion of the lower side roll 2, and the rotation shaft 7 is supported by two bearings 11 and 9, which are arranged to be adjacent to each other in an axial direction. A housing 12, which is positioned at an inner side and which contains the bearing 11, is fixed to the frame 6 through a main cylinder 13. On the other hand, a housing 10, which is positioned at an outer side and which contains the bearing 9, is fixed to the frame 6 through a cylinder 8 for bending. Also, a gap adjusting means 14, which is used to adjust a gap .delta. between the rolls 1 and 2, is provided between the housing 5 and the housing 11.
In order to adjust the gap .delta. between the rolls 1 and 2, a wedge 15, which is provided in the gap adjusting means 14, is displaced by use of a driving device (not shown) in an axial direction of the roll (direction of an arrow c). Together with the above operation, the roll 2 is pressurized to the direction of the roll 1 (direction of an arrow b) by the main cylinder 13.
In order that the roll 2 is curved against the roll 1 before forming the plastic sheet, a load is applied to the bearing 9 supporting the end portion of the rotation shaft 7 in a direction opposite to the main cylinder 13 (direction an arrow a) by use of the cylinder 8 for bending. Thereby, bending moment acts on the roll 2 based on the loads to be applied through the bearings 11 and 9 in opposite direction to each other.
As mentioned above, in the conventional roll bending device for forming the plastic sheet, the loads of the direction opposite to each other, that is, pressing force due to the main cylinder 13 and drawing force due to the cylinder 9 for bending are made to act on the rotation shaft 7, individually. Due to this, two kinds of cylinders, the main cylinder 13 and the cylinder 8 for bending are needed. Moreover, since the load of the main cylinder 13 must be changed in accordance with the change of the bending moment on the rotation shaft 7 applied by the load of the cylinder 8 for bending, there are disadvantages in which the control system of the device becomes complicated, and operability worsens.